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Instead of Christmas cards this year, I’m sending out a class-action apology to all those primary students I led so seriously astray with my misguided whiteboard talks. If you’re an instructor, you’ve surely done the same during the standard foreplay before committing lift.

But the lift explanation always includes—at least mine did—what Bernoulli actually discovered, which is that when a fluid—liquid or gas—accelerates, its pressure drops. This has remained a bullet point of accepted physics for almost 300 years now and since we’re living in the same universe, it shouldn’t have changed.

Well, not according to Professor Holger Babinsky at the august Cambridge University, who posted a YouTube video asserting that aeronautical engineers, aerodynamicists, designers and instructors have been getting it wrong—or at least explaining it wrong—for years. Actually, this isn’t a new presentation nor are the assertions found within exactly groundbreakers, either. But the video recently resurfaced on social media and I’ve been getting pummeled by it.

Says the accompanying article, even Albert Einstein reputedly screwed this up in believing that the air on top of the wing accelerates because it has farther to go than the air on the bottom of the wing. See the analysis in the video. The debunking part is that Babinsky claims we’ve been saying for years that the air accelerates because the top camber forces it to travel farther. We have? If I ever used this explanation, I don’t recall it and a cursory look through some of my aviation reference materials doesn’t flog this explanation, either. In his classic Stick and Rudder, Wolfgang Langewiesche boldly said forget Bernoulli entirely and that the wing keeps the airplane up by pushing the air down. That’s actually a Newtonian explanation and knocks a leg out from the straw man erected in the video because it doesn’t mention sentient molecules having a meeting at the leading edge and agreeing to reconvene at a specified time at the trailing edge. (But they gotta rush off; might hit traffic around those protruding rivets at two-thirds span.)

Since my Ph.D. is in light signals not aerodynamics, I referred the video to my friend David F. Rogers, who is an actual aerodynamicist and has taught same to would-be naval aviators at the Naval Academy for many years. Any validity to the video’s claims?

“This is basically nonsense,” he says of the video. “I’ll assume that you know what a stagnation point is and that the flow decelerates to zero at a stagnation point. Knowing that, look at the video again. Notice that there is a stagnation point on the bottom surface. So, the flow in the smoke line that passes around the bottom surface has to decelerate to zero and then accelerate again to some velocity. That takes time. Is there a stagnation point on the upper surface? No.,” he explains.

Look closely at the smoke lines and you’ll notice that they’re more tightly spaced on top of the wing than on the bottom, meaning they’re moving faster. We haven’t translated the worm hole yet, so as Bernoulli postulated, the pressure is lower on top of the wing and higher on the bottom. This has been proven empirically and why it is so is buttressed theoretically with something called the Kutta condition. It explains how the air cleaves at the leading edge, accelerating across the top surface. This is neither exotic nor difficult to understand, but read further here.

The straw man part of the video’s argument is the claim that so many assert that parcels of air on the upper and lower airflow must reach the trailing edge of the wing at the same time, having been cleaved at the leading edge. Supporting straws are that this entirely explains lift. But of course, it’s more complicated than that and if you’re a wiser man than me, you’ll say as much and move on to clicking on how shocked you’ll be at how Loni Anderson looks today.

But … no.

At this juncture, the discussion devolves to tastes better/less filling. “This is like the argument between the physicist and the aerodynamicist/engineer. The physicist approaches the problem from a fundamental change in momentum viewpoint,” says Rogers. That’s the Langewiesche argument that there’s a downward deflection of the airstream that creates a reaction force in the vertical direction that translates to lift and drag.

“The physicist has applied Newtonian physics in its purist form. S/he then claims that the force is notáa result of a pressure change, Ó la Bernoulli, and concludes that the aerodynamicist does not know what s/he is talking about. Of course, this is also nonsense. They are both coming to the same conclusion but using different approaches. Both are using the same Newtonian physics,” Rogers says.

So why get sucked into this click bait in the first place? Because in Don Quixote’s day, he had to ride three days just to find a lousy windmill to tilt. But on the internet, gimbal-mounted windmills come at you like fifth-grade dodgeball and after one has bounced off my forehead five times, it’s time to act. If I hear the music, I’m gonna dance.

I’m waiting on tenterhooks for Professor Babinsky to tackle ADS-B. Then we’ll really be having fun.

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Comments (54)

NASA spent a fair amount of time and effort working on this, as one might expect. And on their educational website they have a whole section devoted to incorrect theories of lift. Since you won't allow me to put links in comments, you can just Google NASA incorrect theories of lift to find it.

Yes, when I was a kid, the first explanation I read for lift used exactly the "straw man" explanation: the camber of the wing means the air going over the top has to take a longer path, so it has to move faster, and then Bernoulli says the pressure drops and holds the wing up. In fact, some versions of the "explanation" went so far as to say that since the bottom of most wings is fairly flat, the pressure there didn't change much, so "most" of the lift was from the "top of the wing".

This "explanation" was bunk.

But, Bernoulli was entirely right. Starting with the Newtonian explanation, we observe that the wing does, in fact, deflect air downward. Now, for the air to be deflected downward as it passes the wing, the pressure above the wing has to be lower than atmospheric pressure, so that air going over the top is deflected downward, toward the lower pressure. And, below the wing, the pressure has to be higher than atmospheric pressure, so that air going below the wing is deflected downward, away from the higher pressure. But, this means the pressure above the wing (lower than atmospheric) is lower than the pressure below the wing (higher than atmospheric), so air approaching the space above the wing will accelerate toward it, while air approaching the space below the wing will slow down - exactly consistent with Bernoulli's rules. And, the velocity difference is often much bigger than could be explained by the wing camber.

I definitely heard the "Equal Transit Time" story from one of my early CFIs, for what it's worth.

Ultimately, it's one of those things that it's not really necessary to explain to students. Why burden them with useless knowledge? As far as things that are interesting to learn more about later, sure, why not, it's interesting enough stuff. But, students are already overwhelmed with how much legal trivia we require of them, aside from the actual operation of the airplane. An extra useless fact means a useful one might get missed.

As far as I'm concerned, conservation of momentum is all the explanation I need. Wing pushes air down and slightly forwards, proportionate to equivalent airspeed and angle of attack. Equal impulse is imparted upwards (lift) and back (drag) into the wing. The actual mechanism by which the wing pushes that air down is just not relevant to the pilot. Knowing the conditions where the wing *stops* pushing air down (namely, stalls) is important. Focus on that.

I heard the "equal transit time" explanation as well, though I can't remember exactly where I heard it. Years ago I found a great website (maybe the above mentioned NASA page) that explained how the wing really generates lift, and the short answer was: it's complicated. The moral of the story was, it's not just one phenomenon but several working together (the exact proportions depending on the type of wing), and to explain it all to a student who just wants to know how to land the thing would digress too far. As a result, we have to simplify the explanation, and as simplifications often do, it can give the wrong impression if taken at face value.

When giving my explanation, I use bernoulli and show how the leading edge of the wing looks like one half of a venturi, so there is a pressure drop on the top of the wing. I also use newton in that the air at the trailing edge of the wing is deflected downward, giving an equal and opposite upward movement of the wing. I then go on to compare a stalled wing to sticking one's hand out of a car window at speed at a 90-degree angle to the wind and how the hand moves backwards instead of up if you had only a slight upward angle. It's not the most precise explanation, but it gives one a basic idea of how a wing generates lift in a manner that is related to the oncoming air in a form that most people are familiar with. And to me, the important part is getting people to think of lift in relation to the relative wind.

If, through forward motion through the fluid, you evacuate a fraction of the air that occupies the space ABOVE a wing (as occurs with any POSITIVE angle of attack, right out to 90 degrees), the evacuated ("wake") area will exhibit a lower pressure than exists elsewhere locally - as in the space BELOW the wing, for example. Higher pressure below; lower pressure above; wing in between. In which direction will the wing want to migrate? "Lift."
Satisfies most pilots; works for all airfoil cross-sections. Compatible with drag, acceleration/stagnation, laminar-flow/separation, Coanda, etc.

This reminds me of the sections that I taught at a local community college, where the most common question was: "why should we care?" The dominant attitude was: "I don't need to know how an automobile engine works, in order to drive a car. Why should I NEED to know how an airplane engine works, in order to fly a plane?" Good question.

"You only really need to know how the engine works when it decides not to."

And even then, you don't really need to know the specifics. Just knowing that an engine needs fuel, air, and an ignition source is all that is really needed. If it's a problem more than that, it's not really something you can do anything about in the air anyway.

Of course, if you're actually interested in learning the real theories of how a wing generates lift or an engine runs, there's nothing wrong with that. It's just not required to safely fly an aircraft, beyond knowing some important concepts like angle of attack, fuel-air mixtures, etc.

- The Bernoulli equation addresses energy conservation in a flowing stream but ignores momentum. As mark says, there is no way a flat-wing balsa glider could "fly" based on energy conservation.

- The momentum conservation equation F=dp/dt ignores the flow energy but does explain the contribution to lift facilitated by the Coanda effect that Tom points out. In the '50s Canada built some air cushion vehicles based on this.

- The third contributor is circulation which comes out of the tensor analysis of fluid flow. My CFD friends have given up trying to make me understand it, but it explains the Magnus effect and why curve balls actually curve.

All three processes seem to be required to explain the load carrying capability of an airplane wing. The Bernoulli principle is probably ascendant because it is the most intuitive.

Seriously though, I personally found Langewiesche's explanation to be the most intuitive because it lays the groundwork for relative wind (aka angle of attack) which is what we all really need to understand as a pilot.

"Just knowing that an engine needs fuel, air, and an ignition source is all that is really needed"

If all aircraft engine were all 21st century technology, I'd agree; just get in and go.
Try telling kids today about carburetor ice and watch them stare blankly until they finally ask "what's a carburetor?". Or leaded gas. Or gravity feed. Or primers. Or manual mixtures. Or why the left mag is not like the right mag. Or even just staring aircraft engines when it's cold weather.

As far as how wings work, isn't it funny how people are now demanding AOA indicators? ;-)

Paul,
Yep, those are the ones I came across a number of years ago (before I got my CFI). Though at some point I must have forgotten about it when I used my "half-venturi" explanation (which was still only meant to be a simplification anyway, though probably not the best one).

"If all aircraft engine were all 21st century technology, I'd agree; just get in and go."

It still comes down to air, fuel, and spark though, at least as far as systems go. Knowing which systems feed the three ingredients to the engine is more important in my opinion than knowing the mechanics of the engine itself.

Ok, so the three NASA briefs all make sense, but that leaves the question: what is the "correct" way to view this? Surely after 115 years somebody knows the answer. Or, should we just go back to looking for Loni Anderson instead?

Speaking of Bernouli vs Newton perspectives ... when you push the stick (steering wheel) forward the airplane doesn't go down, the houses just get bigger ... and vice versa.

Many moons ago when I was teaching a class at night at Edwards AFB and one of the subjects du jour was lift, I came to this same finding. I dove into the books in the tech library all afternoon, drove myself nuts and finally gave up. So it's funny to read this discussion here. Can't we all agree that it's JFM ??

So what's the story on the fixed horizontal stabilizer without a curved upper surface moving through that same fluid?

As far as understanding engines, how does a GDI model work without a venturi ?

As I recall from my college aerodynamics courses in engineering school, there are about 6 ways to calculate lift, all of them giving the same answer. The thing is, you don't use all of them, just pick one, otherwise the answer is six times what it should be. Bernoulli was still right, though.

Now don't forget the wing downwash angle, epsilon (sure wish my keyboard had that Greek letter). That's what some people use to refute Bernoulli. But the Bernoulli effect is what causes this downwash angle. Picture a flat bottom wing with the bottom parallel to the air movement and any camber (think Clark Y). It still creates lift. And it will create different amounts of lift with different upper surface curvature.

Paul - perhaps this is a UK thing. I was certainly taught by my instructors (both rotary and fixed-wing) that the reason for the lower pressure above the wing was that the air had further to travel over the upper camber. The clear explanation given to me (and illustrated in the training materials) was that the particles of air split and rejoin. That explanation is now debunked - I mean that's all, isn't it? Why on earth would anyone call that debunking "basically nonsense"?

Students should be asked to test the idea that lift comes from a curved wing section. The can take their C172 out to the runway, give it full power, and keep their hands flat in their lap and see if the plane lifts off before running out of runway. That will test the faith of "true believers" in Bernoulli at about the 2/3rds mark down the runway...

I used run a pseudo wind tunnel test of flat panel lift when I used to fly the T-34 with the canopy open. I found that muscle force required to keep your arm from being ripped from its socket with an outstretched open hand began at about six inches outboard and was directly proportional thereafter (dF / dx ) to the distance it stuck out.

Ever watch a bird as it glides through the air? Take a guess which way the upper set of feathers flutter. If you look at air to air pictures of light airplanes you can see definition of ribs in the upper surface of the wing where the skin flexes as a result of Bernoulli effect.

How about an airliner taking off on a humid morning. The lower pressure above the wing condensing the moisture in the air. And if airflow above the wing was not the main creator of lift then why do operators and the airlines spend so much money and effort deicing the top surface of the wing prior to takeoff in winter weather? I have never seen the bottom of a wing sprayed with deice fluid!

Uh oh ... a new conundrum just hit me. If some sort of curved surface causes a Bernouli event ergo lift ... how the heck do some of those very weird box looking kites produce lift ?? Maybe strings have lift, too ? Is that the string theory John McNamee was referring to?

" if airflow above the wing was not the main creator of lift then why do operators and the airlines spend so much money and effort deicing the top surface of the wing prior to takeoff in winter weather?"

I can sleep easier now knowing the air on the upper wing speeds up and does not meet the air at the same time at the trailing edge.

However, in our politically correct society, this would indicate the air passing the upper, curved portion of the wing is superior to that passing underneath by virtue getting behind the wing in faster, therefore superior fashion. Ergo, for the sake of equality and the fact that the air beneath the wing is really trying hard to be equal, we came up with an "equality" theory. The air traveling over the top speeds up and then slows down to meet the slightly slower, therefor inferior lower air, to make it feel good...with both arriving at the trailing edge at the same time. This results in "happy" air.

A big thank you to the three "B's" for demystifying and bravely postulating how lift is really produced in a politically correct atmosphere. Three cheers for the courageous Bernoullii, Babinsky, Bertorelli explanation of lift !

"It has been 13 years since this article appeared in Physics Education. The original aim was to correct some very common misunderstandings resulting from a widespread, but wrong, popular science explanation for lift. While nothing I presented in the article was new science (lift has been well understood by aerodynamicists for centuries), nor was I the first person to point out that the 'popular' explanation was wrong, I nevertheless welcomed the opportunity to contribute this article because the IoP is a fantastic platform to reach a wide audience, particularly in education. The response has been overwhelmingly positive and I still receive regular emails from readers across the globe. More importantly, the article has provided the basis for many lectures I have given to students, teachers, physics societies and at University outreach events. This has been a fantastic opportunity to promote STEM subjects and introduce my favourite branch of science, aerodynamics. Also, tailoring the message to such a wide variety of audiences has deepened my own understanding of the fundamentals of my field - and it never gets boring."

Gentlemen ... aka S/he's ... after nearly 40 comments, I believe we have now achieved aerodynamic Nirvana (not to be confused with the grunge rock group).

We have thus far discussed Bernouli, Newton, Babinsky and Einstein; fluids and gases; lift, drag, stalls and relative wind; humidity, condensation and deicing; acceleration, deceleration and stagnation; the Clark-Y airfoil, wing shape and camber; worm holes; empirical vs. theoretical analysis; particle vs. wave theory; high, low and atmospheric pressure; conservation of momentum; 'steering' wheels controlling airplanes; morals and phenomenon; how an OTTO cycle engine works ... or doesn't; the coanda effect and the Kutta condition; transit time and venturis; vortex shedding and a dog named Vortex (who sheds); mystical (JF) magic; six ways to calculate lift and the 11 dimensions of string theory; money; the faith of true believers ... and much, much more. In the end, we've all agreed that air can be made "happy" so I guess THAT provides lift? It's necessarily simplistic because mere mortals -- and even pilots -- have limits of understanding and finite amounts of time.

We've watched a YouTube video by Babinsky and read his abstract. We've re-read Kirschner and Langewiesche and Don Quixote and checked in with Rogers at the Naval Academy ... just to be sure. I, myself, re-read 'Private Pilots Book of Aeronautical Knowledge.'

We've even discussed using hyper-aspirin therapy to accelerate achieving the afterlife where -- apparently --- Burt and the seraphim use happy lift to fly box kites while they await Loni.

I now decree that anyone who has actively participated in this subjet d'jour has now achieved the honorary title of Doctor of Aerodynamic Nonsense ... DrAN. Some of us may now hope to Profess our knowledge on the unwitting or gullible. Millenials and ERAU two stripers will undoubtedly drink from our fountain of koolaid.

Doctors! It's Friday and it's raining ... anyone up for pizza? Somewhere in the environs of Sarasota there must be an establishment with a table for 24 that has ... are ya ready ... jimno piwo? (Google it)(Raf knows). Professor Emeritus Paul can provide the invocation.

What's Stencil cookin' up - a Skype party with alcohol? I think I need a bottle of DrANital. More efficacious than the customary dose of screwitall.
And anchovies with my toxic romaine; not on my pizza.
Send selfies.

The most fundamental mechanism of lift is so much less sexy and utterly useless in regard to understanding the design of a wing. But the truth is the near countless individual molecules of air impacting the lower surface of the wing impact it with more aggregate force than their counterparts on top of the wing. That is how the air imparts a force on the wing, the individual molecules of aluminum in the wing skin have no telekinetic knowledge of the greater flow field.

Beyond that all of the other heartfelt arguments are more based on what equations we decide to use, and where we setup the boundary box when we sketch out the physics models.

"Just knowing that an engine needs fuel, air, and an ignition source is all that is really needed"

Just knowing that Lance Armstrong moves his legs up & down and his hands side to side is NOT all that's you really need to know to ride a bicycle through the Alps at speed.
Just knowing that you blow into one end of a flute and move your fingers over some holes along the other end does is not enough to join a symphony.

Flying a plane at speed means you have to know more than the simplistic explanation you saw in a 3rd grade picture book. Flying is a performance sport and you'd better be on your best competitive level each time you launch. The NTSB reports are full of pilots names who "knew all about" flying....

"Now can I hear it from the extreme right?"
(Summons up best Robert DeNiro immitation) Are you talkin' to ME?

I agree that you never can know too much about your vehicle or about flight in general. But that's not at all the same as advocating for a requirement that a pilot know X, Y, or Z. In my worthless opinion, at least 50% of what we shovel into pilots' snot-filled heads is of no practical value whatever; it comprises yet another dis-incentivising barrier-to-entry for the typical pilot-wannabee.

Example: magneto impulse couplings. You can't even begin an intelligent discussion about that obscure topic unless it is preceded by a working understanding of crankshafts, camshafts, spark plugs, and the mysterious concept of "top dead center." Scoff if you want to, but in 2018, most wannabees think that "suck, squeeze, burn, blow" is the punchline to an off-color joke. Fact is, there is nothing a pilot in the cockpit can do to to adjust the timing of a magneto. Nothing. With apologies to Keebler, they might as well be baked in Magic Ovens.

You can't know too much. But you can be required to know far more than you NEED to know, to get the job done. Sometimes you don't need to know how the watch was made; you just need to know what time it is.

" Fact is, there is nothing a pilot in the cockpit can do to to adjust the timing of a magneto."

Fact is..... you need to know when to turn them OFF if they (as they do) go wrong.
Knowing that you CAN'T adjust them means that you'll need to be of the CORRECT mindset to turn them OFF in order to live. ;-)

Mark:
Politely, knowing enough to try running the engine on ONE magneto (it's on the rough-engine checklist) has NOTHING at all to do with understanding the function of an impulse coupling. Turn 'em on; turn 'em off is the only recourse available in the cockpit. THAT was my point.